Intermolecular Forces Essay

There are many different types of intermolecular forces. These forces may be classified under the following arbitrary but convenient headings.
i. Electrostatic forces ii. Induction forces iii. Dispersion forces iv. Specific (chemical) forces Intermolecular forces are always expressed in terms of potential energy functions. The common convention is that a force of attraction is negative and force of repulsion is positive.

i) Electrostatic Forces Intermolecular forces, due to point charges are called electrostatic forces. The potential energy between two ions can be written as (1.22) where Zi and Zj are ionic valences and Dr is the dielectric constant of the medium. Electrostatic forces between ions are inversely proportional to the square of the separation and therefore they have a much longer range than other intermolecular forces which depend on higher powers of the reciprocal distance. These

In the fields of moderate strength, the induced dipole moment µi is proportional to the field strength, E as follows. µi = E (1.23) where is the proportionality factor. When a non polar molecule ‘i’ is situated in an electric field set up by the presence of a nearby polar molecule ‘i’ the resultant force between the permanent dipole and the induced dipole is always attractive. The mean potential energy can be calculated by using (1.24) Polar molecule as well as non polar molecule can have dipoles induced in an electric field. For molecules with a dipole moment the potential energy due to induction is usually small when compared to the potential energy due to permanent dipoles and similarly, for molecules with a permanent quadrapole moment, the induction energy is less than that due to quadrapole – quadrapole